The present invention relates to a road surface frictional coefficient estimation device which estimates a frictional coefficient of a road surface on which a vehicle travels, a driving force distribution control device that controls driving force distribution between front and rear wheels of a four-wheel-drive vehicle, and a four-wheel-drive vehicle.
In the related art, there has been a four-wheel-drive vehicle which determines whether or not a road is a low μ road based on a rotational difference between front and rear wheels of the four-wheel-drive vehicle, and controls driving force to be transmitted to the rear wheels side based on the determination result (for example, see Patent Document 1).
The four-wheel-drive vehicle disclosed in Patent Document 1 includes tight corner braking preventing means for avoiding a tight corner braking phenomenon during turning with a large steering angle by reducing the engaging force of a clutch transmitting the driving force to the rear wheels, low μ road determination means for determining whether or not a road is a low μ road, and low μ road means for not reducing the engaging force of the clutch when it is determined that the road is a low μ road.    (Patent Document 1) JP-A-2003-312289
However, since a rotational difference between the front wheels and the rear wheels is also generated by the difference between track followed by the front and rear inner wheels during turning, the determination whether or not the road is a low μ road can be accurately performed only while traveling straight. That is, in a case of traveling straight, due to the difference between the driving force of the front wheel and the driving force of the rear wheel, the front wheel to which the great driving force is transmitted compared to the rear wheel easily slips, and it is possible to estimate the frictional coefficient of the road surface based on the rotational difference between the front wheels and the rear wheels. However, since the rear wheel passes further on the inside compared to the front wheel during turning, if the frictional coefficient of the road surface is calculated based on the rotational difference due to the difference between track followed by the front and rear inner wheels during turning, there is a possibility that a high μ road is erroneously determined as a low μ road.
In order to prevent such an erroneous determination, for example, a method of determining whether or not a vehicle travels straight with reference to a steering angle of a steering device, and determining the frictional coefficient of the road surface based on the rotational difference between the front wheels and the rear wheels when determining that the vehicle travels straight, is considered. However, depending on the type of the steering device, after the ignition is turned on and the traveling is started, it is not possible to decide a neutral position (an angular position of a steering wheel during traveling straight) of the steering angle for a time period and thus it is not possible to accurately detect the steering angle. This is a specific problem in a steering device to which a relative angle sensor having a relatively simple configuration capable of detecting the speed and the direction of the steering is applied rather than an absolute angle sensor capable of detecting an absolute value of the steering angle.
That is, in this type of the steering device, since it is possible for a steering operation to be performed in a state where the engine is stopped after the ignition has been turned off, while the ignition is on, the neutral position is decided based on a traveling state each time when the ignition is turned on without using information of the steering angle before the last time when the ignition is turned off, and thus it is not possible to accurately obtain information of the steering angle until the neutral position is decided.
In such a steering device, based on detection values of a yaw rate and a transverse acceleration of a vehicle, it is determined whether or not the position of the steering wheel corresponds to a straight traveling state of the vehicle, and the steering angle at a time point when the position of the steering wheel corresponds to the straight traveling state of the vehicle is stored as a neutral position. Accordingly, after the traveling is started, until the vehicle enters a stable straight traveling state and the neutral position of the steering angle is decided, it is not possible to determine whether or not the vehicle travels straight based on the steering angle. For this reason, the estimation of the frictional coefficient of the road surface also can not be performed, and, for example, when there is a need for turning with a large steering angle immediately after starting, as a vehicle exit time from a parking lot, there is a case where it is not possible to perform the suitable control depending on the frictional coefficient of the road surface.